// Problem: 
// Contest: 
// URL: 
// Memory Limit: 
// Time Limit: 
// #pragma GCC optimize("Ofast")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#include<bits/stdc++.h>
using namespace std;
#define int long long
inline int read(){
	int s=0,w=1;
	char ch=getchar();
	while(ch<'0'||ch>'9'){if(ch=='-')w=-1;ch=getchar();}
	while(ch>='0'&&ch<='9') s=s*10+ch-'0',ch=getchar();
	return s*w;
}
signed main()
{
	int t=1;
//	cin>>t;
	while(t--)
	{
		string a,b;
		cin>>a>>b;
		int l=a.size(),s=0;
		for(int i=0;i<l;i++)
		{
			if(a[i]!=b[i])
			{
				a[i]=a[i]=='o'?'*':'o';
				a[i+1]=a[i+1]=='o'?'*':'o';
				s++;
			}
		}
		cout<<s;
	}
	return 0;
}
